Turn on your Wii U, login to the user's profile that has the Mii you want to transfer, and go to System Settings.

Go to amiibo Settings, then hit Register Owner and Nickname. Touch the amiibo to the GamePad as directed. Change the Owner if needed then hit save. Touch the amiibo to the GamePad again to register the owner.

Hop over to your Nintendo Switch and load up the System Settings. Scroll down to User then select Add New user. Once you're given the prompt to choose an icon scroll to the Mii tile and click hit.

Choose Create New Mii, then select Copy Mii from amiibo. Grab the amiibo you used before and touch it to the NFC touchpoint on the right Joy-Con. Your amiibo should be successfully copied. After it's transferred from the amiibo you can customize the Mii's pose, expression, and completely edit the original Mii.